On-sky demonstration of astrophotonic fiber Fabry-Pérot correlation spectroscopy

High sensitivity spectroscopy of astronomical targets is used for determining stellar radial velocities, exoplanet detection, and even exoplanet atmosphere sensing. However, high resolution spectrographs are bulky, highly complex and expensive instruments. While this bulk optical approach is versatile, fiber optic photonic instruments can be lower cost, more compact, and simpler to parallelize for multiple targets. Here we present a low-cost fiber-based correlation spectroscopy technique which can be used for simultaneously measuring radial velocity and molecular/atomic composition of astronomical targets. The correlation is achieved using a commercial, piezoelectrically tunable fiber Fabry-Pérot (FFP) filter that can be tuned from 1520 to 1620 nm. The output of the filter is measured using a single channel photodetector and processed using a lock-in amplifier. By adjusting the bias and modulation amplitude of the transmission spectrum of the FFP filter, the device can be optimized for maximum sensitivity to a certain absorption/emission line. We perform an on-sky demonstration using a 4.25 cm telescope to detect telluric CO2 with the sun as a background light source.